With reference to FIG. 16, a disk rotor 5 is fixed to an outer end surface of a flange 10 of a hub wheel 2 by means of a plurality of bolts 12 which are inserted into through holes 11 formed in a circumference of the flange 10. The disk rotor 5 is fixed to the flange 10 after a bearing device for axle 1 is attached to a drive shaft 4. Therefore, it is necessary to previously attach the bolts 12 into the through holes 11 of the flange 10. The bolts 12 are attached into the through holes 11 in non-separable manner so that the bolts 12 do not come out the flange 10 when the bearing device for axle 1 is independently handled. For this reason, the bolt 12 is formed with a serration 13. The bolt 12 is fitted into the through hole 11 in such a state where the serration 13 bites into a circular inner peripheral surface of the through hole 11.
In this state, a diameter of the through hole 11 is finely increased by its plastic deformation, and an outer end surface of the flange 10 is finely undulated in its circumferential direction and radial direction. As a result, a surface precision of the outer end surface of the flange 10 is lowered. More specifically, an inner diameter of the through hole 11 is expanded by μm unit in the serration 13 of the bolt 12 as shown with arrows in FIG. 17, and the surface precision is lowered. The deterioration of the surface precision in the outer end surface of the flange 10 causes the deterioration of adhesion of the disk rotor 5 with respect to the flange 10, whereby the disk rotor 5 is inclined and its rotation precision is disadvantageously deteriorated.
Therefore, it is an object of the present invention to provide a bearing device for axle capable of stabilizing the attachment position of the disk rotor with respect to the hub wheel.